It is customary to use integrated optical devices in the case where, for example, it is desired to have a luminous beam carried by an optical fiber passed to another optical fiber. The simplest case of such a transfer consists of carrying out an end-to-end connection of two optical fibers. It is also possible to transfer all or part of the energy transported by inserting an integrated optical device between two optical fibers.
The simplest device shown on FIG. 1 consists of a guidance structure which comprises a thin guidance layer 10, known as a core, inserted between two layers 20, 30, all of which is laid onto a substrate 40. The core refraction index or coefficient is always greater than the refraction coefficients of the adjacent layers 20, 30.
The luminous beam spreads into this guidance layer 10 with one dying out part in each of the adjacent layers. FIG. 1 shows a cross section along the direction of propagation.
In such a structure, an electromagnetic field may spread according to two types of propagation modes: a first type of mode, known as a transversal or quasi-transversal mode (noted TE), the electrical field associated with the electromagnetic wave for this type of mode being in the plane of the guidance layer, and a second type of mode, known as a magnetic transversal or quasi-transversal mode (noted TM) where the magnetic field is within the plane of the layer. Generally, the integrated optical devices are embodied so as to only allow for propagation of a single mode of each type; these modes are known as fundamental modes. In the remainder of the description and for the sake of more simplicity, the text shall consider the case of structures only allowing for propagation of these fundamental modes. Any wave inclined with respect to these two modes is of necessity split into two components, one being of type TE, the other being of type TM.
One known integrated optical device makes it possible to carry out transfers by coupling. FIG. 2 shows a cross sectional view of such a circuit perpendicular in the propagation direction and FIG. 3 shows a top view of said circuit. The structure of the integrated optical electromagnetic wave coupler shown in these figures comprises two generally identical layers 3, 4 between which two out-of-joint layers 1,2 are placed with an index higher than those of the layers 3 and 4 and identical for the two layers, these layers occuring on a given plane, as can be seen from the section shown on FIG. 2. The layers 1 and 2 associated with the adjacent layers 3 and 4 form parallel optical guides. The entire unit is laid down on a substrate 5. The choice of the interval e between the two layers, each forming the actual cores 1, 2, enables action to be taken on the coupling coefficients KE and KM between the two guides. These coefficients are also a function of the parameters characterising the guides. KE and KM respectively represent the coupling coefficient of the modes TE and TM (guided modes).
When an electromagnetic wave is injected into the core guide 1, all the energy of this wave is able to pass into the core guide 2 at the end of propagation distances, namely functions of the coupling coefficients KE, KM, provided the propagation speeds of the guided modes of the same type are identical in each of the guides taken independently. In the case where the propagation speeds are not equal, the energy transfer shall be partial, but shall reach a maximum at the end of distances which are functions of the coupling coefficients KZ and KM, but also of the difference between these speeds.
Conventionally, guided structures are used having a small index difference n between the core and the two layers between which it is inserted, as the guidance structure is consequently scarcely dependent on the type of wave polarization. This is a distinct advantage and in fact it is possible to guide a luminous wave with such a structure without being too much concerned with its polarization, the TE and TM modes then having almost the same field profiles and effective indices (note that the effective index is the ratio between the speed of the light and the speed of the guided mode). Moreover, this type of structure allows for high coupling efficiency between a monomode optical fiber and the fiber-optical light guides.
Now, it has been proved necessary in certain applications to use guidance structures with slight index differences (slight n) so that at least one of the guides is slightly sensitive to polarization and possesses sound connection efficiency with a monomode fiber and, at the same time, is able to separate the polarized components from the guided electromagnetic field. This is the case when, for example, it is sought to embody a polarization diversity receiver and also when it is sought to embody optical sensors using luminous polarization.
None of the existing integrated optical devices are slightly sensitive to the polarization of a luminous wave, as well as allowing for separation of the two polarized components TE and TM.
The present invention makes it possible to resolve this problem. It concerns a coupler type integrated optical device which, by virtue of its special structure and optical characteristics, makes it possible to obtain the two desired functions.